Process for chromium tanning and bath for carrying out said process

ABSTRACT

TANNING SKINS FOR MAKING LEATHER, BY TREATING THE SKINS IN A BATH CONTAINING A BASIC CHROMIUM SALT. CHROMIUM TANNING IS EFFECTED WITH A MIXUTRE OF TRIVALENT CHROMIUM SALT AND A DOUBLE CARBONATE OF ALUMINIUM AND AN ALKALINE METAL, PREFERABLY SODIUM. AN ALUMINIUM AND SODIUM SALT IS PREFERABLY USED, OBTAINED BY THERMAL TREATMENT OF A DOUBLE CARBONATE ALUMINIUM AND SODIUM FOR 2-3 HOURS AT 250-300*C., HAVING THE FOLLOWING BASIC FORMULA, THE PERCENTAGES BEING EXPRESSED BY WEIGHT:   PERCENT AL2O3 30-50 CO2 10-25 NAI2 10-20   THIS PROCESS PROVIDING HIGH QUALITY TANNED ANIMAL SKINS.

United States Patent 3,690,812 PROCESS FOR CHROMIUM TANNING AND BATH FOR CARRYING OUT SAID PROCESS Andre Klein, Marcq-en-Bareul, France, assignor to Societe Anonyme: Ugine Kuhlmann, Paris, France No Drawing. Filed Feb. 3, 1970, Ser. No. 8,431 Claims priority, appliscgagizosizfrance, Feb. 5, 1969,

Int. Cl. Q14-c 3/04, 3/06 US. Cl. 894.27 7 Claims ABSTRACT OF THE DISCLOSURE Tanning skins for making leather, by treating the skins in a bath containing a basic chromium salt.

Chromium tanning is effected with a mixture of trivalent chromium salt and a double carbonate of aluminium and an alkaline metal, preferably sodium. An aluminium and sodium salt is preferably used, obtained by thermal treatment of a double carbonate aluminium and sodium for 2-3 hours at 250-300 C., having the following basic formula, the percentages being expressed by weight:

Percent A1 0 30-50 CO 10-25 Na O 10-20 This process providing high quality tanned animal skins.

For the tanning properly speaking, a more or less basified chromium salt is used, generally chromium sulphate. The basicity of a chromium salt corresponds to the percentage of the main valences of chromium atoms which are saturated with hydroxyl groups. It is generally given in Schorlemmer degrees. A salt in which each chromium atom is combined with an OH group has a basicity of 33.3 Schorlemmer degrees; if two OH groups are combined with each chromium atom, the basicity of the salt is 66.6 Schorlemmer degrees. The higher the basicity of a chromium salt the higher its tanning capacity. Also, its affinity for the skin is greater the more alkaline is the latters reaction.

As a general rule, based on experience, tanning should be progressive. If it is too sudden large amounts of the tanning compound are fixed on the surface, it only penetrates the skin with difiiculty, and leather so treated has serious defects: raised grain, hollows of chromium, difficulty in obtaining even dyeing and poor distribution of fatty materials in the feed.

Pickling is carried out to avoid these defects; it is used to decrease the aflinity of the skin for chromium salt by acidifying it; the skin pH, which is about 8 after deliming,

3,690,812 Patented Sept. 12, 1972 is lowered to a value nearer that of the chromium salt tanning solution, i.e. a pH in the range of 1.5 to 3.2. Acidification is generally carried out in presence of sea salt to avoid any swelling which might damage the skin.

Similarly, to obtain progressive tanning, the chromium salts used in tanning are generally of low basicity. In the most conventional process, chromium sulphate having a basicity in the range of 30 to 45 Schorlemmer degrees is used. The reaction of a very acid skin with a chromium salt having a basicity in this range results in insufficient tanning, which has to be completed. To effect this, the operation known as the basification step is carried out during tanning: when the chromium salt has completely penetrated the skin, a basic compound (sodium carbonate or bicarbonate, borax), is added to the tanning bath, thus increasing the basicity of the chromium salt absorbed by the skin, or still present in the bath, and therefore its tanning capacity.

If on the other hand, basicity is increased, the amount of Cr O fixed is increased, but the solution becomes more astringent and the skin has a wrinkled grain side, tanning in depth is less satisfactory.

The prior art has shown that the addition of neutral salts such as sodium sulphate, sea salt, or alkaline salts of organic acids, such as formic, phthalic, oxalic acids, etc., decreases the astringency of the tanning bath, and to a certain extent enables the operation to be carried out at a substantially higher basicity.

Similarly, in industrial practice, it is often necessary to increase the basicity to neutralize the effect of the acid retained in the skin, in accordance with the intensity of the acidic treatment (pickling) which preceeds tanning. If this acidic treatment is carried out in the presence of suitable masking products (formate, phthalate, oxalate, etc.) it is even possible to effect the pickling steps, then chromium tanning, in the same vat, using a very basic chromium sulphate.

To sum up, the drawbacks of the prior art, in as far as fixing the acidity arising from the hydrolysis of trivalent chromium salts with sodium carbonate is concerned, are as follows:

(1) The sodium carbonate precipitates the chromium hydrate which cannot thereafter be put back into solution:

(2) Uneven tanning;

(3) Constant supervision throughout the entire process is indispensable.

Among the documents relating to the prior art the following may be mentioned: German Pats. 229,030 and 389,418 which, respectively, provide for the addition of calcium carbonate to chromium sulphate, and the addition of water insoluble magnesium derivatives such the magnesium carbonate to the tanning liquid. French Pat. No. 1,379,548 also provides for the use of dolomite as an acid-fixing substance during tanning with trivalent chromium salts. Finally, French Pat. No. 1,466,426 may be cited, which also describes a process wherein pickling was carried out in the presence of a synthetic tannin.

The object of the invention is a chromium tanning process which overcomes the drawbacks of the prior art with respect to processes using basification.

The tanning process with chromium salts proposed by the invention, which uses acid fixing substances, consists in that chromium tanning is carried out with a mixture of a chromium salt and a double carbonate of aluminium and an alkaline metal.

In practice, the proportion of the double carbonate of aluminium and an alkaline metal in the mixture according to the invention varies from about 8 to 12% by weight based on the trivalent chromium salt, notably chromium sulphate.

A double carbonate of aluminium and sodium, which is easily available in practice is preferably used.

The basic composition, calculated as oxides, double carbonate of aluminium and sodium is advantageously as follows, the percentages being given by Weight:

Percent A1 30-50 CO -25 Na O 10-20 Before mixing the double carbonate of aluminium and alkaline metal with the chromium salt, and particularly with highly basic chromium sulphate, said double carbonate is preferably subjected to a thermal treatment at moderately high temperatures, notably in the range of 250- d 300 C., for several hours, two to three hours for example.

This thermal treatment converts the starting double carbonate at least partially into alkaline oxides and aluminium oxide. It has been found that the use of such thermally treated double carbonate gave practical advantages in tanning. In fact, thermally treated double carbonate has a lower reactivity than the original double carbonate, which can be advantageous, especially at the beginning of tanning treatment. The thermally treated double carbonate reacts with the acids, freed during the hydrolysis of trivalent chromium salts, more slowly than untreated double carbonate.

The invention also relates to composition for tanning animal skins, particularly in the form of aqueous tanning solution comprising a mixture of trivalent chromium salt and a double carbonate of aluminium and an alkaline metal, preferably sodium.

Such compositions can further comprise other acid-fixing substances.

The main advantages provided by the invention for tanning animal skins are as follows:

(a) economy of labour (b) simplification of the tanning process (e) no possibility of the formation of a too strongly basic precipitate (d) the production of high quality leathers.

The invention is illustrated, without being limited, by the following example.

EXAMPLE The leathers obtained are of excellent quality, with very fine grain sides, without wrinkles or shrivelling and, what is more, without hollows.

What is claimed is:

1. In a process for tanning animal skins with at least one water-soluble basic trivalent chromium salt as tanning agent using acid-fixing substances, the improvement consisting in carrying out chromium tanning with an aqueous composition consisting essentially of a mixture of at least one basic water-soluble trivalent chromium salt and 8 to 12% by weight based on the trivalent chromium salt of a double carbonate of aluminium and an alkaline metal subjected to thermal treatment in the range of 250300 C. for about two to three hours before mixing said double carbonate with the chromium salt.

2. The process of claim 1, wherein a double carbonate of aluminium and sodium is used.

3. The process of claim 2, wherein the basic double carbonate of aluminium and sodium composition used is as follows, the percentages being given by weight:

Percent 1880 30-50 2 10-25 Na O 10-20 4. A composition for tanning animal skins consisting essentially of a mixture of at least one basic water-soluble trivalent chromium salt and 8 to 12% by weight based on the trivalent chromium salt of a double carbonate of aluminiurn and an alkali metal subjected to thermal treatment in the range of 250-300 C. for about two to three hours before mixing said double carbonate with the chromium salt.

5. The composition of claim 4 wherein the said double carbonate is double carbonate of aluminium and sodium and the said chromium salt is trivalent chromium sulphate.

6. The composition of claim 5 comprising an aluminium and sodium salt having the following basic formula, the percentage being given by weight:

Percent A1 0 30-50 CO 10-25 Na O 10-20 7. An aqueous bath for tanning animal skins consisting essentially of an aqueous solution of the composition of claim 4.

References Cited UNITED STATES PATENTS 2,301,637 11/1942 Niedercorn 1495 3,423,162 l/ 1969 Papayannis 894.25 3,232,696 2/1966 Heyden et a1. 8-94.29

GEORGE F. LESMES, Primary Examiner J. P. BRAMMER, Assistant Examiner US. Cl. X.R. 

